The wetted length is the length of the three-phase boundary line for contact between a solid and a liquid in a bulk third phase. Due to the surface tension, a force, which acts on the solid at an angle θ (contact angle) between the solid surface and the liquid surface, acts at every point on this boundary line (three-phase point).
The wetted length is used in the calculation of the surface tension or interfacial tension in the Du Noüy ring method, the Wilhelmy plate method and in the rod method. Furthermore, it must be known when measuring the contact angle using the Wilhelmy method in order to calculate the contact angle from the wetting force with a liquid with known surface tension.
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